FIG. 1 illustrates a communications base station 10. The communications base station 10 comprises a radio equipment controller (REC) 15, which may also be referred to as a baseband unit, and a radio equipment (RE) 20, which may be referred to as a remote radio unit. The radio equipment 20 is coupled to a radio antenna 25. Although not shown in FIG. 1, the radio equipment controller 15 is associated with a first clock, and the radio equipment 20 is associated with a second (different) clock. The radio equipment controller 15 and the radio equipment 20 may be connected by one or more communications links, such as optical fibres.
In this example, the radio equipment controller 15 and the radio equipment 20 communicate according to the CPRI (Common Public Radio Interface) protocol. The CPRI protocol requires that the propagation delay between the radio equipment controller 15 and the radio equipment 20 is predicted. The CPRI protocol assumes that the downlink propagation delay (i.e., with reference to FIG. 1, T12, the time it takes a signal to travel from REC 15 to RE 20) is the same as the uplink propagation delay (i.e. with reference to FIG. 1, T34, the time it takes a signal to travel from RE 20 to REC 15). Thus, the CPRI protocol calculates the propagation delay between the radio equipment controller 15 and the radio equipment 20 as the round trip delay of a signal between the radio equipment controller 15 and the radio equipment 20 (i.e. with reference to FIG. 1, T14), minus any known processing delay at the radio equipment 20 (i.e. Toffset), divided by two.
However, in practice, the uplink propagation delay may not be the same as the downlink propagation delay (i.e. the links may be asymmetric). This may be the case where the uplink and downlinks travel over respective communications links, for example over respective optical fibres, which have different lengths. This may also be the case in WDM (wavelength division multiplexed) networks, where the uplink and the downlink may travel through the same optical fibre (over different wavelength channels) but through respective add/drop optical filter arrangements, which may introduce asymmetries.
The asymmetry between the uplink/downlink may be calculated, by determining the propagation delay of each of the links manually. However, this process is time consuming and costly, particularly since a communications base station 10 may comprise several RECs 15 and several REs 20, each located at remote locations.